PROJECT SUMMARY (Project 2) Metastatic castrate resistant prostate cancer (mCRPC) in bone is almost universally fatal. While new targeted therapeutics have improved patient survival, resistance invariably develops. Treatment refractory bone metastases lead to morbidity and mortality in patients with mCRPC. Long-term goals of this proposal are to understand mechanisms of therapy resistance and strategies to overcome them. Studies of this proposal focus on resistance mediated from the bone microenvironment. Specifically, using an osteogenic prostate cancer xenograft, MDA-PCa-118, we found that treatment with cabozantinib, an oral multi-kinase inhibitor with potent activity against p-MET and p-VEGFR-2, demonstrated striking initial responses. However, resistance rapidly occurs, as first evidenced by viable cells tumor cells found in proximity to newly formed bone matrix. Because prostate cancer bone metastasis presents with a unique bone-forming phenotype, we hypothesize that factors released from tumor-induced bone lead to pre-existing resistance, in which the microenvironment has already contributed factors that mediate resistance prior to application of therapy. To study the mechanisms of this form of resistance, a secretome analysis was performed on conditioned medium from prostate cancer-induced bone, which identified 121 bone-secreted proteins. Many of these bone-secreted proteins activate integrins through paracrine effects, increasing tumor cell survival. We term these osteoblast-secreted paracrine factors ?osteocrines?. Consistent with the involvement of integrins in therapy resistance, we found that FAK, the downstream effector of integrin signaling, is highly activated in therapy-resistant tumor cells. Thus, we hypothesize that osteocrines released from prostate cancer-induced bone form a pre-existing resistance niche that mediates therapy resistance of prostate cancer cells through activation of FAK. We will test this hypothesis by: (1) Examining the ability of selected osteocrines to confer therapy resistance through activation of FAK; (2) Examining the effects of second-generation FAK inhibitors (VS-6063 or VS- 4718) on overcoming osteocrine-induced therapy resistance in xenograft mouse models; and (3) Conducting a clinical trial to examine the toxicity and efficacy of a FAK inhibitor (VS-6063 or VS-4718) in men with treatment- refractory bone-metastatic castrate-resistant prostate cancer. The studies would be paradigm shifting by demonstrating that the tumor microenvironment can provide a niche of ?pre-existing resistance?, a mechanism of resistance in addition to tumor adaptation to therapy, which is likely applicable to multiple therapies in mCRPC. The studies would also demonstrate the potential efficacy of FAK inhibitors for treatment of bone-metastatic CRPC. In addition, delineating mechanisms of this ?pre-existing resistance? will provide new predictive markers to guide therapeutic strategies to overcome resistance.